A microfluidic MEMS device is formed by a plurality of ejection cells each having a fluid chamber; an actuator chamber; a membrane having a first surface facing the actuator chamber and a second surface facing the fluid chamber; a piezoelectric actuator on the first surface of the membrane; and a passivation layer on the piezoelectric actuator. The membrane has an elongated area defining a longitudinal direction and a transverse direction. The passivation layer has a plurality of holes. The holes extend throughout the thickness of the passivation layer and, in a plan view, have an elongated shape with a greater dimension parallel to the longitudinal direction of the membrane and a smaller dimension parallel to the transverse direction.